Strange Cubase behavior related to RAM usage

When Cubase uses more than approximately 4GB of RAM, loaded, in my case, by samples into HALion, I noticed that trying to change note heads in Score Edit makes the whole Cubase GUI become unresponsive. I can’t click anywhere inside windows, only the menus and already opened VSTi windows work. Unless I unload some samples to get below 4GB memory usage, and everything works again.

Any idea anyone ? Can someone reproduce ?

Edit :
I also tried in Windows 7 64bit, Cubase 7.5 and HALion 5 : same behavior but the limit is around 6GB loaded into RAM.